⚡Electric Charge & Current

The fundamental building blocks of electricity — charge, current, and how electrons flow through conductors.

What is Electric Charge?

Electric charge is a fundamental property of matter, like mass. There are two types: positive (carried by protons) and negative (carried by electrons). The fundamental unit is the elementary charge e = 1.602 × 10⁻¹⁹ C (Coulombs). One electron carries exactly -e. Like charges repel with force F = k·q1·q2/r² (Coulomb's Law), opposite charges attract.

Total charge = number of carriers × elementary charge (1.602\times 10^{-19}C)

Conductors, Insulators & Semiconductors

Conductors (copper, aluminum, gold) have loosely bound outer electrons that move freely — electron sea model. Resistivity ≈ 10⁻⁸ Ω·m. Insulators (rubber, glass, ceramic) have tightly bound electrons — resistivity > 10¹² Ω·m. Semiconductors (silicon, germanium) have intermediate conductivity that can be controlled by doping, making them the foundation of modern electronics.

Electric Current

Current is the rate of charge flow: I = dQ/dt. Measured in Amperes (A) — 1A = 1C/s flowing through a cross-section. Conventional current flows from + to - (Benjamin Franklin's convention). Electron flow goes from - to + (actual physical movement). In circuit analysis, we use conventional current. A 60W light bulb at 120V draws 0.5A. The USB-C standard can deliver up to 5A at 20V (100W).

Current = Charge/Time | Also: carrier density × area × drift velocity × charge

⚠️ Safety Thresholds

1 mA: perception threshold (tingle). 5 mA: slight shock. 10 mA: muscle contractions, can't let go. 30 mA: respiratory paralysis possible. 100 mA: ventricular fibrillation likely. > 2A: severe burns, cardiac arrest. Always use GFCI/RCD protection in wet areas.

🔌 Real-World Currents